ASVAB Auto Information — Vehicle Electrical System and Battery Questions

Cold-start problems can come from many sources, but A/C failure is NOT among them — A/C is for cooling the cabin, has nothing to do with starting. Cold weather affects starting in several ways: (1) BATTERY CHEMISTRY slows in cold — chemical reactions producing current are temperature-dependent; at 0°F, a battery has only 60% of its 80°F cranking power; (2) ENGINE OIL THICKENS — viscosity rises in cold; thicker oil is harder to pump and creates more friction in bearings, requiring more cranking effort; multi-grade oils (5W-30) help — the 'W' rating is the cold viscosity; lower numbers flow better cold; (3) FUEL VOLATILITY DECREASES — gasoline doesn't vaporize as readily when cold; harder to ignite; modern winter-blend gasoline has more volatile components added; (4) STARTER MOTOR STRAINED — turning thick oil takes more current at the moment battery is weakest; (5) ELECTRICAL DEMANDS UP — heated seats, defrosters, heaters, lights all on at startup, drawing current that could go to starting; (6) DIESEL CONCERNS — diesel fuel gels in extreme cold (waxy components solidify); winter-blend diesel has anti-gel additives; very cold weather requires fuel heaters or block heaters; (7) GLOW PLUGS — diesels use glow plugs to preheat combustion chamber; weak glow plugs prevent cold start. Cold weather starting tips: (1) Use BLOCK HEATER (electric heater that warms engine block, oil pan, or coolant) for prolonged cold; common in northern climates; plug in for a few hours before starting; (2) DON'T pump accelerator (older carbureted cars only) — modern fuel-injected cars don't need this; (3) Keep BATTERY in good condition — replace before getting very weak; test annually; (4) Use correct OIL VISCOSITY for climate (manufacturer's recommendation accounts for typical climate); some recommend SAE 0W or 5W oils for very cold climates; (5) USE WINTER-GRADE FUEL — automatic for most US stations in winter, but watch in transitional weather; (6) KEEP FUEL TANK MORE FULL — reduces moisture condensation that can freeze in fuel lines; (7) PARK INSIDE if possible — even unheated garage is warmer than outside; (8) GIVE IT a moment — modern cars start almost immediately, but very cold cars may take a few extra seconds of cranking; alternate cranking 5-10 seconds then waiting 30 seconds (don't damage starter from continuous cranking). Battery age: typical lifespan 3-5 years in mild climate, 2-4 years in extreme climates. Replace before failure. JUMP STARTING: with cables and a running car (or jump-starter); (1) Connect RED to dead battery POSITIVE; (2) RED to good battery POSITIVE; (3) BLACK to good battery NEGATIVE; (4) BLACK to a GROUND on dead car (engine block, frame — NOT to dead battery negative; prevents spark near battery hydrogen gas); (5) Let donor run a few minutes; (6) Try to start dead car; (7) Disconnect in REVERSE order. Modern jump starter packs are safer (built-in protection). Don't try to push-start a vehicle with automatic transmission or a modern fuel-injected engine — won't work.

Top Dead Center (TDC): the position of a piston at the highest point in the cylinder, where it momentarily stops before reversing direction downward. The volume in the cylinder is at its MINIMUM at TDC (combustion chamber volume). Bottom Dead Center (BDC): the lowest piston position; volume at MAXIMUM (combustion chamber + displaced volume). These reference points are essential for engine timing and measurements. STROKE: distance piston travels from TDC to BDC (and same back). On crankshaft, the piston is at TDC when crank pin is at TOP of its rotation circle; at BDC when at BOTTOM. Piston moves slowly near TDC and BDC (changing direction); fastest at mid-stroke. BTDC (Before TDC): position is approaching TDC on compression stroke; ignition timing usually specified BTDC (e.g., '10° BTDC' means spark fires when piston is 10° of crank rotation before TDC); ATDC (After TDC): after passing TDC on power stroke. VALVE TIMING events relative to TDC/BDC: (1) IVO (intake valve opens) — typically a few degrees BTDC on exhaust stroke to start opening before piston reaches top; (2) IVC (intake valve closes) — typically a few degrees ABDC on compression stroke, after piston has started up but inertia still drawing in air; (3) EVO (exhaust valve opens) — typically BBDC (before bottom dead center) on power stroke; (4) EVC (exhaust valve closes) — typically ATDC on intake stroke. VALVE OVERLAP: brief period when both intake and exhaust valves are open simultaneously, around TDC at end of exhaust / start of intake; helps with cylinder scavenging at high RPM but can hurt low-end performance. Performance engines have more overlap; passenger car engines have less. CAMSHAFTS DETERMINE VALVE TIMING — different cam profiles (duration, lift) optimize for different RPM ranges. PERFORMANCE CAMSHAFTS have higher lift and longer duration, sacrificing low-end for high-end. VARIABLE VALVE TIMING (VVT/VVT-i/VTEC/VANOS etc.) — adjusts cam timing electronically/hydraulically to optimize across RPM range; common modern technology. IGNITION TIMING: when the spark plug fires relative to piston position; typically 10-30° BTDC at idle; advances (fires earlier) with RPM (giving combustion time to develop pressure as piston is at top); advances with engine demand. Excessive advance causes KNOCK/PING (premature ignition); too retarded causes power loss and overheating. KNOCK SENSORS detect knock and ECU retards timing automatically. IGNITION TIMING is often diagnosed with a TIMING LIGHT (strobe synchronized with #1 cylinder spark) aimed at timing marks on the crankshaft pulley/harmonic balancer. Many modern engines run distributorless ignition with timing entirely controlled by ECU based on sensor inputs.

Drive systems: (1) FRONT-WHEEL DRIVE (FWD) — engine drives only front wheels; usually mounted transversely (sideways); engine + transmission combined in TRANSAXLE (combined transmission and differential); driveshafts (CV axles) connect transaxle to front wheels. PROS: more interior space (no driveshaft tunnel to rear); lighter; better fuel economy; better traction in snow (engine weight over driving wheels); cheaper to manufacture; less expensive to maintain. CONS: torque steer (engine torque pulling steering); less ideal weight distribution; understeer in performance driving; harder to make very high-power vehicles (front tires steer and drive). Used by: most economy and family cars; minivans; compact SUVs. (2) REAR-WHEEL DRIVE (RWD) — engine drives only rear wheels; engine typically mounted longitudinally (front-to-back); driveshaft from transmission runs under vehicle to rear differential; rear axles to wheels. PROS: better weight distribution (engine front, drive rear); better handling balance; no torque steer; allows more aggressive performance driving; better for heavy towing/hauling (weight transfer to drive wheels under acceleration); easier to drive separate trailer; can handle higher power (no steer-and-drive conflict); preferred by most performance car enthusiasts. CONS: less traction in snow/ice (drive wheels have less weight); driveshaft tunnel reduces interior space; more parts (driveshaft, rear differential); heavier; less fuel efficient. Used by: sports cars (Mustang, Camaro, BMW, Corvette); luxury cars (most BMW, Mercedes-Benz, Lexus full-size, Cadillac); pickup trucks (RWD base, 4WD optional); some SUVs; semi-trucks. (3) ALL-WHEEL DRIVE (AWD) — engine drives all four wheels; CENTER DIFFERENTIAL (or transfer case with viscous coupling, electronic clutch, etc.) splits power between front and rear axles; front and rear differentials split power between each axle's wheels. Typically OPERATES CONTINUOUSLY (full-time AWD) or AUTOMATICALLY ENGAGES when slip detected. PROS: best traction in all conditions (snow, rain, mud, dry); better acceleration; more confident handling; safer in adverse weather. CONS: more complex, heavier, more parts to maintain/repair, lower fuel economy, more expensive. Used by: Subaru (Symmetrical AWD on most models); luxury cars (most AWD options on BMW, Mercedes, Audi); SUVs and crossovers; performance cars (high-power applications); pickup trucks (4WD versions). (4) FOUR-WHEEL DRIVE (4WD or 4x4) — similar to AWD but typically DRIVER-SELECTABLE; 4-HIGH for normal driving on slippery surfaces; 4-LOW for severe off-road (very low gearing for crawling); often LOCKED CENTER DIFFERENTIAL (or simply mechanically connected front-rear) — works on loose surfaces but binds and damages drivetrain on dry pavement; OPTIONAL LOCKING DIFFERENTIALS for axles (off-road). Used by: off-road and serious work vehicles (Jeep Wrangler, Toyota 4Runner, pickup trucks with 4x4 option). AWD vs 4WD distinction is blurring in modern usage; many systems combine elements. ELECTRIC VEHICLES: each wheel can have its own motor (or single motor + differential); typically dual-motor AWD (one for front axle, one for rear); torque vectoring (varying torque to individual wheels for cornering) is easier with separate motors. Performance/handling: weight distribution and drive layout fundamentally affect how a vehicle handles; this is why car enthusiasts often have strong preferences (RWD for spirited driving, AWD for all-weather usability, FWD for everyday utility and economy).

Radiator pressure cap: critical component that pressurizes the closed cooling system. Functions: (1) PRESSURIZES the system — typically 13-17 psi above atmospheric pressure (varies by vehicle); rating stamped on cap; (2) RAISES BOILING POINT of coolant — each 1 psi of pressure raises water's boiling point about 3°F; at 15 psi, water's boiling point rises from 212°F to about 257°F; antifreeze additive further raises boiling point; pressurization prevents coolant from boiling at high engine temperatures (engine can run at 230-240°F without boiling); (3) PRESSURE RELIEF VALVE — releases excess pressure if system overheats (preventing burst hoses or radiator); pressurized coolant escapes to OVERFLOW RESERVOIR; (4) VACUUM RELIEF VALVE (in newer caps) — when engine cools and coolant contracts, a vacuum could form; vacuum relief allows coolant to flow BACK from reservoir to fill the system; prevents reservoir overflow when system warms again; this is essentially a closed coolant recovery system. SAFETY WARNING: NEVER REMOVE RADIATOR CAP WHEN HOT — pressurized hot coolant can erupt out, causing severe burns. Wait until engine has cooled completely (1-2 hours minimum after running); or if necessary, cover cap with thick rag and slowly turn 1/4 turn to release pressure gradually before fully removing. Cap can lose its seal or pressure rating over time; replace as needed (about $10-20 part); failed cap symptoms: coolant loss with no visible leak (boiling off through relief), white deposits around cap (escaping coolant), overheating without other obvious cause. Aftermarket high-pressure caps for performance applications: 18-22 psi; raise boiling point further; race applications. OVERFLOW RESERVOIR (expansion tank): translucent plastic bottle next to radiator; typically marked with COLD and HOT level lines; receives expanding coolant when engine warms; returns coolant when engine cools; should be at appropriate level. Some modern vehicles use a DEGAS BOTTLE (different design where reservoir is the system's primary fill point, radiator cap may not exist). CHECKING COOLANT LEVEL: ONLY when COLD; check reservoir level mark; if low, add 50/50 premix antifreeze (matching color/type — never mix incompatible coolants). Modern coolants: ORGANIC ACID TECHNOLOGY (OAT, extended life, 5+ years), HYBRID OAT (HOAT), INORGANIC ADDITIVE TECHNOLOGY (IAT — traditional green); using wrong type can cause corrosion. SIGNS of cooling system problems: visible leaks under car; sweet smell of antifreeze; steam from hood; high temperature gauge reading; coolant warning light; oily coolant (head gasket leak); coolant in oil (head gasket leak — milky 'chocolate milk' oil); white exhaust smoke (coolant entering combustion chamber). HEATER CORE: small radiator inside dashboard providing cabin heat; failure causes coolant leak inside cabin (sweet smell, fog on windshield, wet floor); replacement is labor-intensive due to dashboard removal. THERMOSTAT: separate component; valve that opens at specific temp (typically 195°F); stuck closed = engine overheats; stuck open = engine slow to warm up, poor heater. RADIATOR FAN(S): electric fans behind radiator turn on as needed; controlled by ECU based on coolant temp and A/C compressor; failure causes overheating in slow traffic but not highway. WATER PUMP: belt-driven (sometimes timing-belt-driven) pump that circulates coolant; failure causes overheating and possible coolant leak; bearing noise or leaking pulley shaft are warning signs; replacement typically with timing belt service in some engines.

Timing belt or timing chain: synchronizes crankshaft (which drives pistons) with camshaft (which operates valves); ensures valves open and close at correct points in the engine cycle. Crankshaft to camshaft ratio is 2:1 (crank rotates twice for each cam rotation), since cam controls a complete 4-stroke cycle which takes 2 crank revolutions. TIMING BELT: rubber/composite reinforced toothed belt; typically lasts 60,000-100,000 miles depending on manufacturer; lighter than chain; quieter; cheaper to manufacture; REQUIRES SCHEDULED REPLACEMENT — when it breaks or jumps, valves and pistons can collide. TIMING CHAIN: metal chain similar to bike chain; typically lasts the life of the engine (200,000+ miles common); noisier; heavier; more expensive; lubricated by engine oil; very rarely fails if oil changes are kept up; modern engines increasingly use chains for the longevity benefit. TENSIONERS: keep belt/chain at proper tension; spring-loaded or hydraulic; replace with belt; chain tensioners may use oil pressure. GUIDES/RAILS: keep belt/chain on path; especially for chains in cam covers. INTERFERENCE vs NON-INTERFERENCE engines: in INTERFERENCE engines (most modern engines for efficiency reasons), valves can extend into space pistons occupy at TDC; if timing belt/chain breaks or jumps, valves and pistons collide — bent valves, damaged pistons, possibly cracked head, scored cylinders; repair often as expensive as engine replacement ($2000-5000+); NON-INTERFERENCE engines have clearance between valves and pistons even when valves are open at TDC; broken belt/chain just stops the engine with no damage; vehicle just needs new belt and re-timing; rare in modern engines. WHEN TO REPLACE TIMING BELT: per manufacturer interval (60K-100K miles); time matters too (rubber degrades — about 7 years even if low miles); replace with associated parts (water pump if belt-driven, tensioner, idler pulleys, seals); SIMULTANEOUSLY because they're behind the belt cover and labor cost is the same. Cost: $400-1500+ depending on engine; not optional on interference engines. Symptoms of failing belt: difficult to detect without inspection; manufacturer interval is the guide; if there's ticking noise, oil leak from front of engine, or visible cracks/wear on belt — replace immediately. TIMING CHAIN problems: stretching over time (rare, usually from oil starvation); broken guides or tensioners; failure has same valve/piston catastrophe risk as belt breakage; usually preceded by rattling noise from front of engine, especially at startup. CAM PHASING (Variable Valve Timing): modern engines have actuators that vary cam position relative to crankshaft (within a range), optimizing valve timing for different conditions; failure causes rough running, check engine light, possibly P0011/P0014/etc codes. PUSHROD vs OVERHEAD CAM: OHV (overhead valve) / pushrod engines (some V8s, older engines): camshaft in BLOCK; pushrods transfer motion to ROCKER ARMS in cylinder head that open valves; uses timing CHAIN typically (short distance). OHC (overhead cam) engines: camshaft(s) in cylinder HEAD directly above valves; longer timing belt/chain; common modern design; DOHC = double overhead cam (separate cam for intake and exhaust valves). Direct-acting valves or with rocker followers/buckets/hydraulic lifters. INTERFERENCE engines: most modern OHC engines; high compression and tight clearance require precise timing. Common interference engines requiring strict belt service: Honda VTEC engines, most modern Asian and European engines, modern turbo engines. Non-interference engines (rare modern, more common older): some Chevy 350s, older Ford V8s, some older 4-cylinders. KNOW YOUR ENGINE — check before assuming, or service belt at recommended interval as if interference.

The thermostat regulates coolant flow to maintain optimal engine operating temperature — it opens when the engine reaches operating temp to allow coolant to flow through the radiator.

Disc brakes use hydraulic fluid pressure to push brake calipers/pads against a rotating disc (rotor), creating friction to slow the vehicle.

The alternator converts mechanical energy (from the engine belt) into electrical energy, charging the battery and supplying power to vehicle electronics while the engine runs.

MPG = Miles Per Gallon, the measure of fuel economy — how many miles a vehicle travels on one gallon of fuel.

The carburetor mixes air and fuel in the proper ratio for combustion in older engines. Modern engines use fuel injection instead, which more precisely controls fuel delivery.